The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device using bend alignment of liquid crystal molecules for effecting display.
In recent years, active matrix liquid crystal display devices have dramatically improved in display performance such as contrast, brightness and viewing angle characteristic, to a level as high as that of cathode ray tubes (CRTs). The liquid crystal display mode currently used widely is a twisted nematic (TN) mode having a response time in the order of tens of milliseconds. With this level of response speed, which is low compared with that of CRTs, blurring occurs when a moving image with vigorous motion such as that in sports is displayed. The causes of this blurring of a moving image were studied in depth by Ishiguro, Kurita and others (Shuichi Ishiguro, Taiichiro Kurita, “Study on moving image quality of hold emission type display with 8X CRT”, ITE Technical Report, IDY96-93, BCS96-23, BFO96-50).
According to the literature described above, the deterioration of the moving image quality of a liquid crystal display device is caused by delay of the response time of a liquid crystal material with respect to the frame rate and by adopting hold type display as the display scheme. The hold type display can be changed to impulse type display by letting a backlight blink or inserting screen erase intervals for the display with liquid crystal. As for the delay of the response time, mainly two measures as follows have been proposed.
One is a surface stabilized ferroelectric liquid crystal (SSFLC) mode using ferroelectric liquid crystal proposed by Clark and Lagerwall in 1980. This mode is however disadvantageous in that the way of gray scale display is complicated and that the alignment stability of smectic liquid crystal is inferior. Due to these and other disadvantages, commercialization of this mode, once started by Cannon Inc., is presently discontinued.
The other is a pi-cell approach using nematic liquid crystal invented by J. P. Bos and others in 1983. In this approach, a voltage is applied across a splay-aligned liquid crystal layer to transform the splay alignment to bend alignment, and a change in retardation with the intensity of the voltage application in the bend-aligned state is used for display. In this display mode, only liquid crystal molecules in the surface portion of the liquid crystal layer move with application of an external field, and a backflow generated in the process of relaxation of the alignment serves to accelerate the response. Due to these features, high-speed response of several milliseconds is reportedly possible.
In the pi-cell approach described above, splay alignment is energetically more stable than bend alignment in the non-voltage applied state. For this reason, some methods for transforming splay alignment to bend alignment have been proposed.
For example, U.S. Pat. No. 6,069,620, discloses a method in which an intense voltage is applied at the start of operation to allow liquid crystal molecules to overpass the energy barrier and thus be transformed to bend alignment. Japanese Laid-Open Patent Publication No. 11-7018 discloses a method in which a high pretilt region is formed on a surface of at least one of a pixel electrode and a counter electrode. Using the high pretilt region as a nucleation site, a liquid crystal layer is transformed to bend alignment, with expansion of disclination, during voltage application.
Japanese Laid-Open Patent Publication No. 9-96790 discloses the following. By adding a chiral dopant to a nematic liquid crystal material so that d/p>0.25 is satisfied (where d is the thickness of a liquid crystal cell and p is a pitch of a spontaneous helical structure of the liquid crystal material with the chiral dopant mixed therein), the liquid crystal layer exhibits 180-degree twisted alignment in the non-voltage applied state. When a voltage is applied to this twisted liquid crystal layer, the twisted alignment is smoothly transformed to bend alignment without generation of disclination.
The three methods described above have been mainly proposed in relation with the transformation of the liquid crystal layer from splay alignment to bend alignment. These conventional methods however find difficulty in attaining sufficiently swift transformation from splay alignment to bend alignment and also reliable transformation over the entire display plane.
The present inventors examined use of a 180-degree twisted region as a nucleation site. However, it was found difficult to form a 180-degree twisted region stably and reliably. In fact, part of the region failed to be 180-degree twisted, but splay-aligned.
An object of the present invention is providing a liquid crystal display device capable of attaining swift and reliable transformation from splay alignment to bend alignment or from bend alignment to splay alignment.